The human spine typically has a characteristic S-shape that, when deformed or injured, presents special problems to solve. The individual vertebrae comprising the spinal column can be displaced from their normal position in any one or more of the three dimensions, referred to herein as the X, Y, and Z axes. The X axis is defined as extending laterally, or side-to-side, across the human body. The Y axis is defined as extending longitudinally, or from head to foot, through the human body. The Z axis is defined as extending transversely, or from front to back, through the human body. Various events can cause deformation: fracture, slippage, tumors, and infections are examples. To correct the alignment of the vertebrae, it may be necessary to compress, distend, or rotate all or part of the spinal column along any one or more of X, Y, or Z axes.
To maintain the vertebrae in the desired alignment, a surgical fusion technique is often indicated. Fusion, however, is not usually successful unless the vertebrae are also fixed for a time period by a mechanical device installed internally during surgery. This allows the fused bone time to heal. Numerous mechanical devices or systems have been made for this purpose. Screw and rod systems and screw and plate systems are commonly used mechanical systems. The former system typically uses a rigid rod secured to the spine by screws inserted in the pedicles for holding the rod. The rod is bent to the desired configuration, and this both manipulates and holds the vertebrae in that same configuration until the fusion process can permanently accomplish the same thing. The latter system uses a rigid plate instead of a rod, and the plate is simply bolted with pedicle screws and nuts to the spinal column to again provide the desired configuration and rigidity to the spine.
Some screw and rod systems, however, suffer several disadvantages. In the lumbar sacral area, the proper curvature or alignment is sometimes lost after surgery because the rod disengages from the screw/rod connectors. Further, each patient has his or her own spinal characteristics or anatomy, including bone shape and density. Thus, the exact location of the system and the pedicle screws, and the exact configuration of the rigid rod, is determined while the patient is on the operating table. Bending the rod consumes operating time, because contouring the rod to correspond to the three-dimensional configuration of the spine can be extremely difficult and ca lead to mistakes.
The three dimensional anatomic orientation of the pedicle is also difficult to ascertain prior to surgery. Thus, screws inserted into the pedicle may frequently need to be re-orientated, and their depth of insertion readjusted. This often requires complete screw removal, particularly if spinal deformation was not exactly as anticipated or if the space available for the screw was insufficient. Removing and replacing screws jeopardizes the fragile bone structure of the pedicle around the screw holes, as well as consuming additional surgical time.
Plate systems suffer some of the same and other disadvantages. Since the plate is normally secured closer to the vertebral bodies than the rod, and the surfaces of the posterior structures of the vertebrae are uneven, more bone must be removed in order to properly position the plate. The plates also have more limited configurations because they are more difficult to bend over a long distance. As with a screw/rod system, the need for adjustment of the height of the screws, and often the removal and reinsertion of screws during the surgical implantation procedure to account for individual peculiarities in bone structure, frequently jeopardizes the integrity of the screw holes and the entire fixation process.
Due to the three dimensional configuration of the spine, the ideal system would be one which can be easily adjusted to this three dimensional configuration. The system would allow correction along the longitudinal (vertical) Y-axis, including compression, distraction, and rotational correction of the vertebrae around the Y-axis, such as seen in scoliosis. This system should also allow correction of the laterolisthesis along the transverse X-axis, as well as rotation around the X-axis which permits correction of kyphosis and lordosis. Finally, correction should be possible for displacement along the Z-axis forward/ anteriorly, known as anterolisthesis, or backwards/ posteriorly, known as retrolisthesis, as well as rotation around the Z-axis. At the present time, a system is not available which can allow the multi-directional, multi-axial correction over multiple bone segments (three or greater). Because of this, present systems-whether they are screw/rod, hook/rod or plate-all have limited usefulness allowing some, but not all needed corrections.
Thus, there is a need for a multi-directional, multi-axial fixation system which can restore the anatomic configuration no matter what the deformity. It should also permit easy manipulation and handling of instrumentation during surgery.